The present invention relates generally to storage and distribution systems for Liquified Natural Gas (LNG). More specifically, the present invention relates to an apparatus and method for condensing boil-off vapor received from an LNG storage tank and condensing the vapor into an output stream for routing into a distribution system.
Imported Liquified Natural Gas (LNG) is stored at many locations throughout the world. The LNG is used when a local source of natural gas is not available or as a supplement to local sources.
Liquified Natural Gas (LNG) is typically stored at low pressure and in liquid form at cold temperatures at an import terminal. The LNG is usually pumped to a pressure that is slightly above the pressure of the natural gas distribution pipeline. The high pressure liquid is vaporized and sent to the distribution pipeline. The pumping operation typically involves a set of low pressure pumps located in a storage container connected in series to a set of high pressure pumps located outside the storage tank.
As is well known, heat input into the storage container generates boil-off vapor. Additional vapor generation may occur during filling of the storage container. Vapor may be obtained from an outside source such as a ship. Ideally, the boil-off vapor is included with the sendout to the distribution pipeline.
Compressors may be used to boost the vapor to the high operating pressure of the pipeline, which can be as high as 100 bar. Compressing the vapor to these high pressures requires considerable energy. A more energy efficient method for disposition of the vapor to the pipeline is desired.
A more energy efficient system utilizes the cold LNG sendout to condense vapor at a low interstage pressure. The vapor condensate combines with the liquid sendout flow and enters the high pressure pumps. The stream flows to the vaporizers from the high pressure pumps. Compressing the boil off vapor stream to the distribution pipeline pressures requires considerably more energy than boosting the boil off vapor condensate to the high pressure with a liquid pump. Several existing LNG import terminals have systems which condense boil off vapor at low pressure and pump the condensate with the liquid stream flowing to the vaporizer. However, the boil off vapor condensers at these prior art terminals lack the physical arrangement and control systems to obtain proper operation and high efficiency.
In accordance with one aspect of the invention, a system for condensing vapor includes the steps of providing contact area within a condenser vessel, directing vapor to the condenser vessel, providing a condensing fluid and a pump fluid, directing the condensing fluid to the condenser vessel, varying the flow of the condensing fluid to control the pressure in the vessel, contacting the vapor with the condensing fluid to create a condensate, and combining the condensate and the pump fluid.
In further accordance with a preferred embodiment, the condenser vessel is utilized as a pump suction vessel. The step of forming a surface layer of liquid that is substantially at its saturation temperature may be included. The condensate and the pump fluid may be mixed in the vessel, or the condensate and the pump fluid may be mixed outside the vessel.
The pressure in the vessel may be controlled by removing vapor during high pressure conditions in the vessel. Alternatively, the pressure in the vessel may be controlled by adding vapor during low pressure conditions in the vessel. Further, the pressure in the vessel may be controlled by removing vapor during high pressure conditions in the vessel and by adding vapor during low pressure conditions in the vessel.
In accordance with another aspect of the invention, a system for condensing vapor includes the steps of providing contact area within a condenser vessel, directing vapor to the condenser vessel, providing a condensing fluid and a pump fluid, varying the flow of the pump fluid to control a liquid level in the vessel, directing the condensing fluid to the condenser vessel, varying the flow of the condensing fluid to control the pressure in the vessel, contacting the vapor with the condensing fluid to create a condensate, and combining the condensate and the pump fluid.
In accordance with a further aspect of the invention, a system for condensing vapor includes the steps of condensing vapor in a condenser vessel to create a condensate, mixing the condensate with a pump fluid to create a combined stream, measuring a temperature difference between the condensate and the combined stream, and utilizing the temperature difference to provide control logic for a vapor compressor.
In accordance with yet another aspect of the invention, a vapor condenser includes a vessel, a liquified gas input to the vessel, control means on the liquified gas input for varying a first liquified gas stream to control the pressure in the vessel, a vapor input to the vessel, means for condensing vapor in the vessel, and means for combining condensed vapor with a second liquified gas stream.
In accordance with a still further aspect of the invention, a vapor condenser includes a vessel, a liquified gas input to the vessel, control means on the liquified gas input for varying a first liquified gas stream to control the pressure in the vessel, a vapor input to the vessel, means for condensing vapor in the vessel, a second liquified gas input to the vessel, second control means on the second liquified gas input for varying a second liquified gas stream to control liquid level in the vessel, and means for combining condensed vapor with the second liquified gas stream.
In accordance with another aspect of the invention, a vapor condenser includes a vessel, means for condensing vapor in the vessel, control means for varying the flow of a first stream of liquified gas to control pressure in the vessel, means for combining the condensed vapor with a second stream of liquified gas to create a combined stream, means for measuring a temperature difference between the condensate and the combined stream, and means for using the temperature difference to control a vapor compressor.
Other features and advantages are inherent in the apparatus claimed and disclosed or will become apparent to those skilled in the art from the following detailed description and its accompanying drawings.